Joking aside a survey of reproductive age heterosexual women and men asked to rank ten characteristics of the opposite sex from most to least important netted the following results:
90% of women ranked how much money a man has as the number one or number two most important male characteristic and
90% of men ranked how good looking a woman is as the number one or number two most important female characteristic.
This explains why there's a beauty industry (and expectation) for heterosexual women but not for heterosexual men. For example women spent more money on cosmetics than we spent on the space program to go to the Moon. It also explains why women's clothing, shoes and accessories are so vastly overpriced compared to men's.
It also explains why the patriarchal glass ceiling has been around forever: men have to have money to get and keep a woman. I once discovered a coworker with the same grade, training and seniority was being paid more because he was married and I wasn't. I asked immediately for a raise from that old boy network and got it.
It also explains why men get paid more than women for the same work.
BTW the list of characteristics was the same for both sexes but did not include penis (or breast) size or any other specific physical characteristic except whether one is "athletic."
In case you were wondering about the opposite score: women ranked how good looking a man is about halfway down the list. Men ranked how much money a woman has about halfway down the list.
As a population study it doesn't say what any one individual is like but it did answer the question "what do most women and men want?"
It also answers the OP's question. Why are most men in relationships? Because most men work and have money.